Thin-Film Flip-Chip (TFFC) LEDs have anode and cathode contacts on the bottom surface, opposite the light emitting surface. Therefore, such TFFC LEDs utilize the whole top die area surface for light output, since top-side (light-output side) metallization for a wire-bond is not necessary. However, with TFFC LEDs, die-level processes, including sapphire substrate lift-off (for GaN based LEDs) and epitaxial layer (EPI) roughening (to improve light extraction), are typically used, which significantly increase the packaging cost. It would be beneficial to not require the sapphire substrate to be removed, while still achieving good light output extraction.
Traditional TFFC LEDs have a rigid dielectric layer formed on the LED semiconductor layers that exposes portions of the p and n-type semiconductor layers. Then, a relatively thin, patterned metal layer (or layers) is deposited over the dielectric layer to directly contact the p and n-type semiconductor layers to create an ohmic contact between the metal and the p and n-type semiconductor layers. Much thicker metal pads (also comprising various metal layers) are then formed over the thin ohmic-contact metal layer(s), such as by plating, for use as the LED's solder pads for mounting the LED to a printed circuit board or other substrate. Solder bumps may then be deposited on the solder pads.
A problem with such traditional TFFC LEDs and processes is that there are stresses between the semiconductor layers, thin metal layer(s), and thick pad layers, such as due to mismatched coefficients of thermal expansion (CTE). Therefore, heat cycling of the LEDs, such as during operation or processing, creates stress that may cause the metal layers to delaminate from each other and from the semiconductor layers, or cause cracking of brittle layers at stress concentration points, which may cause failure. It would be beneficial to provide a technique to reduce such stress to increase the reliability of the LEDs.
Another problem with traditional TFFC LEDs and processes is that the surface on which the thick solder pads is formed is not planar. Therefore, it is difficult to obtain flat solder pads of uniform thickness. Flat solder pads with uniform thickness are beneficial for electrical and thermal conduction between the LED and the printed circuit board or other substrate. It would be beneficial to provide a technique to planarize the “bottom” surface of the LED structure prior to depositing (e.g., plating) the solder pads.